TinEye is an exciting and new image search engine from Idée Inc. The service is currently in private beta, but I managed to garner an invite to use it.
The interface of the site couldn’t be simpler. All you have to do is upload an image or an image URL, and the engine takes care of the rest. It can find images similar to the ones you have uploaded, even if they have been cropped, resized or even photoshopped.
TinEye does this by first creating a digital signature or fingerprint of the uploaded image, and then comparing it to the fingerprint of every other image in its search index.